1. Field
Invention relates generally to tagged magnetic resonance imaging, and in particular to three-dimensional quantification without long image acquisition times and user-intensive post-processing methods.
2. Related Art
The use of magnetic resonance imaging (MRI) for the quantification of regional function of the heart based on the measurement of motion has great potential for clinical adoption. However, the primary limiting factors to date are the lengthy image acquisition protocols and tedious post-processing procedures required to yield regional motion measures.
Three main MRI protocols that have been used for the quantification of myocardial motion are myocardial tagging, displacement encoding with stimulated echoes (DENSE), and phase contrast (PC) velocity encoding techniques. See, generally, Aletras et al., “DENSE: Displacement Encoding with Stimulated Echoes,” Cardiac Functional MRI. J. Magn. Reson. 173 (1999) 247-252; Aletras et al., “Mixed echo train acquisition displacement encoding with stimulated echoes: an optimized DENSE method for in vivo functional imaging of the human heart,” Magn. Reson. Med. 46 (2001) 523-534; Pelc et al., “Phase contrast cine magnetic resonance imaging,” Magn. Reson. Q. 7 (1991) 229-254, and Pelc et al., “Quantitative magnetic resonance flow imaging,” Magn. Reson. Q. 10 (1994) 125-147.
In tagging, myocardial spins are modulated at end-diastole in a pre-specified pattern. Later in the cardiac cycle, the displaced taglines are imaged and tracked using post-processing algorithms in order to compute displacement and strain images. This technique permits rapid imaging and visualization as well as fast, automatic computation of in-plane (i.e. two-dimensional) motion measures using harmonic phase (HARP) processing. See, generally, Osman et al., “Cardiac Motion Tracking Using CINE Harmonic Phase (HARP) Magnetic Resonance Imaging,” Magn. Reson. Med. 42(6) (1999) 1048-1060, and Osman et al., “Imaging heart motion using harmonic phase MRI,” IEEE Trans. Med. Imag., 19(3) (2000) 186-202. To date, however, there has been no extension to three dimensions in an equally efficient and automatic way.
Phase contrast imaging adds to every myocardial spin a phase value proportional to the velocity in the encoding direction. PC imaging times are generally long and phase distortion leads to significant measurement errors. Also, since velocity rather than displacement is the measured quantity, computation of displacement and strain (as opposed to strain rate) at later times in a sequence is typically corrupted by numerical integration errors. PC is readily extended to three dimensions though imaging time becomes prohibitively long.
DENSE encodes position in a manner similar to MR tagging through the use of stimulated echoes. Automatic processing analogous to HARP can then be used to compute displacement and strain. The acquisition protocol of DENSE supports higher spatial resolution than that of conventional HARP techniques, but the computation of in-plane motion is sensitive to through-plane motion in DENSE, unlike conventional tagging techniques.
To date, extension of these three basic approaches to three dimensions has required extensive additional data collection over that of 2-D imaging. Furthermore, except for PC, the results yield only sparse motion information. In all three cases, long imaging times may be prohibitive due to patient breath-holding constraints or may produce sub-optimal results due to gross mis-registration of images collected over a long period of time.
As a result, no practical fully three-dimensional approach to the imaging of regional cardiac function is available. Accordingly, a need exists for imaging techniques and processing methods that overcome these difficulties.